The present invention relates to a receive only, earth satellite ground station and more particularly to low cost, tuner and IF sections for frequency agile receivers, said tuner being able to select at least 24 different channels at the command of the user.
As the cost of integrated circuit chips has reduced dramatically and the availability of special function circuits produced for mass markets has increased, the overall costs associated with even highly complex electronic equipment has been reduced such as to open up markets not contemplated just a few years ago. One such market is private earth satellite communications systems having only a few transmit-receive stations and a plurality of receive only stations. Such systems are presently being contemplated by religious organizations and some of the larger industrial concerns for transmitting voice and picture religious services in the former case and instructional materials in the latter case.
Also, as the costs of such receivers are further reduced, the home TV market begins to become attractive and a test of this market is currently being conducted in Canada.
There are presently two types of commercial satellites in orbit which are used for TV relay. They are the Western Union Satellites and the RCA Satellites. While both operate in the same general frequency bands, the Western Union Satellites have twelve transponder bands each 40 MHz wide and each having a center frequency separation of 40 MHz, while the RCA Satellites have 24 transponder bands each 40 MHz wide, but separated by 20 MHz. RCA is able to accomplish this by utilizing frequency re-use or cross polarization. Basically, this configuration consists of 12 transponders having their signals transmited in the vertically polarized mode and 12 horizontally polarized with each adjacent transponder signal being transmitted in the oppositely polarized mode. The system of the present invention is concerned with the RCA system since it provides 24 channels of communications at substantially no additional cost over the Western Union system at least at the receiver.
Continuing with a description of the basics of either system, the satellite receives a nominal 6 GHz signal transmitted from the uplink terminal, translates it to a nominal 4 GHz signal and retransmits it back to the ground in a beam shaped to cover the geographic area of interest, for instance, the United States.
The signal transmitted back to the ground is received by the down link terminal which is the part of the communications chain of interest in the present invention. The down link chain consists primarily of an antenna and one or more low noise amplifiers, receivers and head end modulators.
Various types of antennas are available, i.e. paraboloidal with feed at the focal point, Cassegrain and conical horn. The paraboloid antenna is of primary concern herein although the conical horn may be employed. The receiver in systems of the type under consideration is critical to system performance. Due to the desire for low cost, relatively small antennas must be employed (5 meters or less) and in consequence, the C/N ratios are not as large as may be desirable. Thus, many requirements are placed on the receiver performance and characteristics that would not otherwise be encountered or required in systems providing higher C/N ratios. If the nominal desirable threshold for an FM receiver with a 30-36 MHz IF bandwidth is approximately 10-11 dB C/N, problems are encountered at the edges of the EIRP Footprint where such a ratio is not available. Some manufacturers employ threshold extension techniques to overcome the problem but they increase system complexity and cost and do not overcome the black and white "sparkle" problem resulting from "impulse noise."